Stretching should never be painful. Go to the point of a gentle pull and STOP! Increase the range of motion as the muscle lengthens; never force it.

But. If stretching is so difficult, it's likely you have a lot of trigger points built up. You can stretch until the cows come home (as my LMT told me) and never loosen the muscle until you work out the trigger points. Get yourself a Thera-Cane or another deep massage tool and a copy of the trigger point book.